An MS/MS mass analysis (or tandem analysis) is known as one of the mass spectrometric methods for identifying a substance with a large molecular weight and for analyzing its structure. A triple quadrupole (TQ) mass spectrometer is a typical MS/MS mass spectrometer. FIG. 11 is a schematic configuration diagram of a generally used triple quadrupole mass spectrometer disclosed in Patent Document 1 or other documents.
This mass spectrometer has an analysis chamber 1 evacuated by a vacuum pump (not shown). This chamber contains an ion source 2 for ionizing a sample to be analyzed, three quadrupoles 3, 5 and 6, each including four rod electrodes, and a detector 7 for detecting ions and producing detection signals corresponding to the amount of detected ions. A voltage composed of a DC voltage and a radio-frequency (RF) voltage is applied to the first-stage quadrupole 3. Due to the effect of the electric field generated by this composite voltage, only an objective ion having a specific mass is selected as a precursor ion from various kinds of ions produced by the ion source 2.
The second-stage quadrupole 5 is contained in a highly airtight collision cell 4. A CID gas, such as argon (Ar) gas, is introduced into this collision cell 4. After being transferred from the first-stage quadrupole 3 to the second-stage quadrupole 5, the precursor ion collides with the CID gas within the collision cell 4, to be dissociated into product ions by a CID process. Since this dissociation can occur in various forms, one kind of precursor ion normally produces plural kinds of product ions with different masses. Then, these plural kinds of product ions are extracted from the collision cell 4 and introduced into the third-stage quadrupole 6. The second-stage quadrupole 5 is normally applied with either a pure radio-frequency voltage or a voltage generated by adding a DC bias voltage to the radio-frequency voltage. Due to this voltage application, the second-stage quadrupole 5 functions as an ion guide for transporting ions to the subsequent stages while converging these ions.
Similar to the first-stage quadrupole 3, the third-stage quadrupole 6 is applied with a voltage composed of a DC voltage and a radio-frequency voltage. Due to the effect of the electric field generated by this voltage, only a product ion having a specific mass is selected in the third-stage quadrupole 6, and the selected ion reaches the detector 7. By appropriately changing the DC voltage and the radio-frequency voltage, it is possible to change the mass of the ion that is allowed to pass through the third-stage quadrupole 6. Based on the detection signals produced by the detector 7 during this operation, a data processor (not shown) creates a mass spectrum of the product ions resulting from the dissociation of the objective ion.
Since, in the mass spectrometer having the previously described configuration, the CID gas is supplied into the collision cell 4, the gas pressure within the collision cell 4 is generally at a few to several mTorr, which is higher than the gas pressure outside the collision cell 4. When an ion travels through a radio-frequency electric field under an atmosphere of such a relatively high gas pressure, the ion gradually loses its kinetic energy due to the collision with the gas, and its flight speed decreases accordingly.
For example, in the case of using an MS/MS mass spectrometer as a detector of a liquid chromatograph, the operation of measuring the signal intensity while sequentially changing the mass of the precursor ion is repeated. In this case, if the flight speed of the ions within the collision cell 4 decreases as just described, it is possible that, when the precursor ion (objective ion) is changed from one ion having a certain mass to another ion having a different mass, the next precursor ion begins to be introduced into the collision cell 4 while the previous precursor ion and the product ions originating from this precursor ion still remain in the collision cell 4, causing these ions to be mixed. This phenomenon is called a “crosstalk” in the MS/MS analysis. The crosstalk may deteriorate the quality of the quantitative measurement of the objective component.
The apparatus described in Patent Document 2 has a linear ion trap with a quadrupole configuration in which a pulsed voltage is applied instead of the ion-capturing radio-frequency voltage to remove ions remaining in the space surrounded by the quadrupole. Due to the effect of the electric field created by the pulsed voltage, the ions are pulled toward the quadrupole and touch the quadrupole to become neutral molecules. However, the radio-frequency voltage applied to the quadrupole is normally as high as a kV-order amplitude; applying a pulsed voltage instead of this high radio-frequency voltage requires a power supply circuit with a rather complex configuration. In fact, the apparatus described in Patent Document 2 uses an elaborate power supply circuit.    Patent Document 1: Japanese Unexamined Patent Application Publication No. H7-201304    Patent Document 2: Pamphlet of International Publication No. 2005/124821